1. Field of the Invention
This invention relates to shallow case hardening process and a post-hardening method of improving the corrosion resistance of shallow case hardened metals.
2. Description of the Prior Art
In the past, it has been a common practice in heat treating of metal work pieces to use fluid beds, such as those made by Procedyne Corporation of New Brunswick, N.J. An example of these fluidized beds is designated 18502048HT, standing respectively for: 1850 degrees F., 20 inch diameter, 48 inch depth. These are, essentially, furnaces that have a sand-like bed, where the sand is made of aluminum oxide. A diffusion plate is underneath the sand, in the sense that the top of a coffee percolator has little holes in it for diffusing water, except that the holes in this case are filled with small screws that are countersunk but not entirely screwed in, and they are oversized holes with respect to the shafts of the countersunk screws, so that a small passageway is created for flow of gasses through the diffusion plate underneath the bed of aluminum oxide.
These fluidized beds are used for carborizing, nitrocarborizing, carbonitriding, and nitro-hardening. In the case of nitro-hardening, temperatures of approximately 1500.degree. F. are used, in an austenitizing type process, to provide core-hardening as opposed to case-hardening of parts. In carborizing, temperatures of approximately 1750.degree. F. are used to provide case-hardening at a high temperature with high carbon content. Nitrocarborizing refers to providing case-hardening with a relatively larger nitrogen content at temperatures of approximately 1050.degree. F. Carbonitriding is provided at temperatures of approximately 1600.degree. F. for a higher carbon content of the mixture of carbon and nitrogen in providing the case-hardening for high Rockwells at surface. While nitrocarborizing is a light case process which occur at low temperature giving high surface hardness without a lot of depth; the opposite is true of carbonitriding which occurs at a higher temperature, and provides a deeper case hardening. The case hardening in carborizing is about 60 thousandths of an inch deep; in carbonitriding it is about 15 to about 20 thousandths of an inch deep; and in nitrocarborizing it is about 3 to about 5 thousandths of an inch deep.
Another process for finishing metal is the Quench-Polish-Quench (or Q.P.Q.) Process for applying corrosion resistance. The Q.P.Q. Process is inadequate because, while providing excellent corrosion characteristics, it destroys the hardening characteristics required, and this has dramatic results affecting tool life and possible failure.
Past experience with the Procedyne Process in the way it has been used provides excellent increases in Rockwell and case-hardening, as opposed to core-hardening. Prior to the subject invention, it had not been considered using processes analogous to the Procedyne Process for achieving not only case-hardening but, simultaneously, corrosion resistance.
Prior to this time, a blanket rule of thumb recommended by Procedyne, and used throughout the industry, was a standard half-hour saturation in the bed at fluid flows of approximately, or at least not exceeding 800 cubic feet per hour. This method employed the following as a standard flow, standard diffusion at the end of the cycle, standard temperature, and standard time within the furnace prior to diffusion: two hours is nitrocarborizing atmosphere, onehalf hour diffusion time, 800 cubic feet per hour total gas flows--its various components are as follows: 35 percent ammonia, 45 percent natural gas, 10 percent nitrogen. This process, while case hardening the treated metal, imparted little or no corrosion resistance.
In certain applications, it is necessary to use metals with even higher degrees of corrosion resistance. Various surface coatings have been proposed for this purpose. Drawbacks to such methods have been many. To date, no process has been found which will yield a metal having increases in Rockwell surface hardness and suitable corrosion inhibitory effects in highly corrosive environments. The corrosion inhibition processes employed heretofore either degrade the case hardened surface or imparts an inhibited surface which wears away quickly under constant use, leaving the metal vulnerable to corrosive forces.
Furthermore, where corrosion inhibiting surface coatings such as conventional paints are applied to conventionally case hardened metals, flaws, imperfections or cracks in the coating will permit corrosive materials access to the metal. It has been found that cracks and the like in an otherwise coated metal will increase the corrosive effects. This can lead to weakening of the part, localized destruction of the case hardened surface and eventual premature part failure.
Thus, it is desirable to provide a process in which a metal surface is case hardened and provided with a high degree of corrosion resistance. It is also desirable that the corrosion resistance and case hardening be achieved in an integral heat-treating process which is uncomplicated and inexpensive. Furthermore, it is desirable to provide a method whereby metal surfaces treated in this manner are coated with a tough long-lasting corrosion inhibiting substances which maintains the integrity and strength of the case hardened surface.